1. Field of the Invention
The present invention relates generally to optical communication systems. More particularly, the invention relates to a cross-connect optical switch for switching optical paths, which is applicable to large-scale switching operation between several hundreds of optical input paths and several hundreds of optical output paths.
2. Description of the Related Art
For example, Wavelength-Division Multiplexing (WDM) optical communication systems require a device for switching a lot of optical signals that have been separated from each other using the wavelength difference and for sending the signals thus switched to their different paths. A “cross-connect switch” is a device to meet this requirement.
In recent years, there has been the growing need to increase the channel capacity in optical communication systems. To achieve this need, the scale of the cross-connect switch has to be enlarged as much as possible. In other words, the cross-connect switch needs to have a capability to switch as many optical input paths as possible to output them to as many optical output paths as possible.
There have been various types of cross-connect optical switch, one of which is designed to switch mechanically the interconnections between optical fibers. An example of this type is disclosed in the Japanese Non-Examined Patent Publication No. 6-208065 published in 1994. This prior-art switch comprises a large number of 1×2 (i.e., one input path and two output paths) mechanically-operated optical switch elements cascade-connected. Each of the 1×2 switch elements switches alternately the input path to one of the two output paths by mechanically shifting the necessary switch element. Thus, this prior-art switch provides the M×N optical-path switching operation.
With the prior-art optical switch disclosed by the Publication No. 6-208065, it is possible to realize the switching operation between several tens of optical input paths and several tens of optical output paths. However, the switching operation between several hundreds of optical input paths and several hundreds of optical output paths is unable to be realized. The reason is as follows.
Specifically, the count of necessary control lines for the mechanically-operated switch elements is proportional to the square of the count of these elements. Therefore, if the prior-art optical switch disclosed by the Publication No. 6-208065 is applied to the switching operation between several hundreds of optical input paths and several hundreds of optical output paths, the count of necessary control lines will be enormous. As a result, the switch size will be too large and at the same time, the switch price will be unrealistically high.
An example of the prior-art cross-connect optical switches has a configuration with optical reflection mirrors for switching optical paths. With the prior-art optical switch of this type, input optical signals are emitted into the atmosphere from optical fibers or optical waveguides and then, they are reflected by mirrors located in the atmosphere to reenter other optical fibers or optical waveguides, thereby conducting a desired optical-path switching operation. Therefore, the above-identified disadvantage that the switch size is too large and the switch price is unrealistically high can be solved. Thus, there is a high possibility to realize a switching operation between several hundreds of optical input paths and several hundreds of optical output paths. However, this has not been realized so far.
With the prior-art cross-connect optical switches using optical reflection mirrors described above, the reflection mirrors for optical path switching are typically driven electrostatically, piezoelectrically, or electromagnetically,
The Japanese Non-Examined Patent Publication No. 11-223339 published in 1999 discloses a cross-connect optical switch, which comprises a silicon substrate, a switching space formed in the substrate, and optical waveguides formed in the substrate in such a way as to be intersected with each other in the switching space A signal light beam emitted formed one of the waveguides is turned to the other waveguide with a displaceable reflection mirror provided in the switching space. The mirror is displaced in the space by the magnetic force generated by an electromagnet provided outside the substrate. In other words, if the mirror is shifted to a specific operation position in the space, the optical signal emitted from one of the waveguides is reflected by the mirror thus shifted and is entered the other waveguide as desired. If the mirror is removed from the operation position, the optical signal is not reflected by the mirror and therefore, it is sent through the same waveguide by way of the space.
When switching the optical paths, a specific pulsed electrical current is supplied to the electromagnet to generate a magnetic force. By the magnetic force thus generated, the mirror is shifted in the space. To make the shift or motion of the mirror(s) smoother, a mirror guide portion is formed in the substrate. This portion is formed by inner walls of the substrate.
With the prior-art cross-connect optical switch disclosed by the Publication No. 11-223778, if the pulsed electrical current is supplied to the electromagnet on the switching operation, the ferromagnetic member of the electromagnet is magnetized. Thus, even if the supply of the electrical current is stopped after the switching operation is completed, the external magnetic field is maintained with the ferromagnetic member thus magnetized. As a result, the switched state of the optical paths is kept unchanged even after the supply of the electrical current is stopped. This leads to reduction of power consumption by the switch.
Furthermore, the Japanese Non-Examined Patent Publication No. 2000-162520published in 2000 discloses a cross-connect optical switch, where optical reflection mirrors are attached to a substrate by way of supports and electromagnets are provided near the mirrors. These electromagnets are located on a holder. Magnetizable elements, which are magnetizable by the magnetic fields generated by the electromagnets, are attached to the mirrors. If a specific electrical current is supplied to a desired one of the electromagnets to generate a magnetic field, the corresponding magnetizable element is magnetized by the magnetic field, resulting in an attractive force between the electromagnet and the corresponding mirror. Due to this attractive force, a two- or three-dimensional displacement of the mirror will occur while holding the mirror on the substrate with the support. On switching operation, a specific pulsed electrical current is supplied to a desired one of the electromagnets to generate a magnetic field, thereby causing a displacement of the mirror in a desired direction by a specific distance.
With the prior-art cross-connect switch disclosed by the Publication No. 2000-162520, like the prior-art cross-connect switch disclosed by the Publication No. 11-223778, even if the supply of the current is stopped after the switching operation is completed, the external magnetic field is kept with the magnetizable element thus magnetized. As a result, the switched state of the optical path is kept unchanged, which reduces the power consumption by the switch.
The Publication No. 2000-162520 discloses an example of a large-sized cross-connect optical switch, which comprises an array of optical reflection mirrors attached to a substrate by way of supports, and an array of electromagnets provided for the corresponding mirrors. With this switch structure, optical signals supplied through an array of optical input paths can be reflected to turn their directions by the mirrors if the desired mirrors are displaced by driving the corresponding electromagnets. Thus, the optical signals can be sent to an array of optical output paths extending in different directions from those of the array of optical input paths.
As explained above, with the optical switch using the optical reflection mirrors for switching the optical paths, there is a high possibility to realize a large-sized cross-connect optical switch. However, with the prior-art switch disclosed by the Publication No. 11-223778, the reflection mirror, which is movably provided in the switching space of the silicon substrate, is selectively displaced in a direction (e.g., in an upper or lower direction) along the mirror guide to insert the mirror into the optical path or remove therefrom, thereby switching the desired path. Thus, there are disadvantages that the mirror guide is essential for stabilizing the displacement of the mirror, and that the mirror guide is located at the intersection of the optical paths. As a result, to realize a large-scale optical switch capable of switching operation between several hundreds of optical input paths and several hundreds of optical output paths, the configuration will be too complicated. This means that the switch structure of the Publication No. 11-223778 is difficult to be adopted for this purpose.
With the prior-art cross-connect switch disclosed by the Publication No. 2000-162520, the array of optical reflection mirrors, which are attached to the substrate by way of the supports in such a way as to form cantilevers, are displaced with the array of electromagnets located on the holder, thereby switching the optical paths. Thus, there is an advantage that the mirrors are displaceable three-dimensionally. However, this advantage will induce a disadvantage that the posture of the mirrors needs to be controlled extremely precisely, and that the posture of the mirrors is unstable unless some contrivance is applied to keep the posture unchanged.
Moreover, if two or more electromagnets are provided for each of the mirrors, the count of necessary control lines for the electromagnets will be twice or more. In this case, there will arise the same problem as explained with respect to the prior-art switch disclosed by the Publication No. 6-208065.
With prior-art cross-connect optical switches formed to drive the reflection mirror electrostatically for optical path switching, there is a problem that electrical charge increase and/or leakage is/are likely to occur, and that the operation is unstable because they are susceptible to environmental conditions.